Restate the problem

They’re going to send me a string of text not longer than 1000 characters. I need to return that string in reverse order with every ‘A’ changed to a ‘T’, every ‘T’ changed to an ‘A’, every ‘C’ changed to a G, and every ‘G’ changed to a ‘C’.

Solution steps

I am not a computer programmer. I’m not trying to become a computer programmer. I enjoy problem solving, technical writing, and bioinformatics. As such, when it’s possible, I’m going to leave the programming to professionals and focus on my interests.

This can be a controversial choice since most of the people who work on Project Rosalind are programmers, so they chose to write their solutions from scratch.

Instead, I’m going to be using code libraries. Code libraries are collections of pre-written code that people use to perform specific tasks more efficiently without having to write their own code from scratch.

Biopython is a massive code library of tools for a wide variety of computational molecular biology tasks. It also happens to be expertly documented.

I’m going to take full advantage of these tools because they’re far more advanced and capable than anything I’m interested in building myself.

Biopython has a function that returns the reverse complement of a DNA strand, so I’m going to use that.

Python concepts

I found the right tool for the job in Biopython.

Once that’s done, including a code library is simple in Python. In my case, I only need:

from Bio.Seq import Seq

You can see all of my code here, but the most interesting line is:

file.write(str(seq.reverse_complement()))

I’ve defined a Sequence, and I’m writing it’s complement to a file.

Bioinformatics concepts

Complementarity describes the relationship between the two sequences of nucleobases on opposite strands of the double-helix.

The relationship is key to the processes of transcription and replication, both of which rely on the properties of complementarity.

A view of the replication process is shown below.

By I, Madprime, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2497221